1. Field of the Invention
The present invention relates to optical waveguide devices incorporating a planar optical waveguide and methods of fabricating the same.
2. Related Background Art
The optical waveguide devices are optical devices with an optical waveguide formed on a substrate, and principal materials thereof are silica glass and silicon. These materials expand or contract with change in temperature and also vary their refractive indices. In consequence, the optical path length of the optical waveguide changes in the optical waveguide device and the effective refractive index of the optical waveguide also changes, thereby leading to variation in optical characteristics.
The conventional, optical waveguide devices were thus equipped with a temperature control system to keep the temperature of the devices themselves constant so as to maintain the optical characteristics thereof constant. This temperature control system is composed, for example, of at least a thermometer, a Peltier device, and a control unit. Namely, in the temperature control system of this type, the control unit controls the Peltier device to heat or cool the optical waveguide device so that the temperature of the optical waveguide device itself, measured by the thermometer, is kept constant.
The inventors investigated the prior arts as described above and found the following problem as a result. The problem was that the temperature control system in the conventional, optical waveguide devices also required a power supply and other elements, in addition to the thermometer, Peltier device, and control unit, and the waveguide devices inevitably became large in size.
The present invention has been accomplished in order to solve the above problem and an object of the present invention is to provide optical waveguide devices having a structure of effectively suppressing the variation in optical characteristics due to temperature change without causing increase in the device size, and methods of fabricating the waveguide devices.
An optical waveguide device according to the present invention comprises at least a first main member having a positive coefficient of linear expansion and a first sub member having a negative coefficient of linear expansion. The first main member has a first major surface and a second major surface opposing the first major surface and a core functioning as an optical waveguide is disposed between these first and second major surfaces. On the other hand, the foregoing sub member is fixed to the first surface of the first main member while covering the entire first major surface of the foregoing first main member.
In the optical waveguide device having the structure as described above, with increase in temperature thereof (e.g., surface temperature), the first main member having the positive coefficient of linear expansion becomes about to expand, while the first sub member having the negative coefficient of linear expansion becomes about to contract. On this occasion, the optical waveguide in the first main member becomes about to prolong its path length because of the expansion of the first main member, while receiving compressive stress from the first sub member because of the contraction thereof. The direction of this compressive stress is parallel to the border between the first main member and the first sub member. Utilization of this difference between the material properties successfully suppresses the variation in the optical characteristics of the optical waveguide in the optical waveguide device due to the temperature change. When the respective linear expansion coefficients of the first main member and first sub member and the respective thicknesses of the first main member and first sub member are properly set, the temperature dependence of optical characteristics of the optical waveguide in the optical waveguide device can be relaxed to a practically negligible level. Since the optical waveguide device can be realized in structure in which the first main member and the first sub member are cemented together, or in structure in which the first main member is directly formed on the first sub member, the size thereof becomes extremely small.
The optical waveguide is normally formed on a substrate, and in the present invention a part of the first main member (e.g., an undercladding), or the first sub member may be the substrate.
The optical waveguide device according to the present invention may have a structure in which sub members having a negative coefficient of linear expansion covers both the first and second major surfaces of the first main member. Namely, the optical waveguide device may comprise a second sub member located so as to sandwich the first main member between the first sub member and the second sub member. In this case, the second sub member has the negative coefficient of linear expansion and is provided directly or through an adhesive on the second major surface while covering the entire second major surface of the first main member. Further, the optical waveguide device according to the present invention may comprise a second main member located so as to sandwich the first sub member between the first main member and the second main member. This second main member has a positive coefficient of linear expansion and is provided directly or through an adhesive on the first sub member while covering an entire major surface thereof opposite to a major surface facing the first major surface of the first main member. In addition, the optical waveguide device according to the present invention may further comprise a third sub member provided directly or through an adhesive on a side face of the first main member, located between the first and second major surfaces, while covering the entire side face. This third sub member has a negative coefficient of linear expansion.
In these cases, the optical waveguide device can be prevented from warping even with change in the temperature of the optical waveguide device itself. Particularly, in the structure wherein the third sub member is fixed around the first main member, even if the optical waveguide device itself changes its temperature because of change in the ambient temperature or the like, the stress on the optical waveguide device will be isotropic on the plane normal to the optical axis of the optical waveguide, thereby effectively suppressing increase in polarization dependence of the optical waveguide in the optical waveguide device.
The optical waveguide device according to the present invention may also comprise a pressure applying structure for applying pressure increasing in proportion to a rise of temperature, in the direction normal to each of the first and second major surfaces of the first main member, in addition to the above various structures. Particularly, the pressure applying structure preferably comprises an insert member having a positive coefficient of linear expansion, and a clamping member for clamping the first main member, sub member, and insert member in the direction normal to each of the first and second major surfaces. This clamping member has a positive coefficient of linear expansion smaller than the linear expansion coefficient of the insert member. In this case, with increase in the temperature of the optical waveguide device itself, expansion of the first main member and contraction of the first sub member causes the optical waveguide in the first main member to be subject to compressive stress in the direction parallel to the first and second major surfaces. In addition thereto, with increase in the temperature of the optical waveguide device itself, the optical waveguide in the first main member is also subject to compressive stress in the direction normal to the first and second major surfaces, because thermal expansion of the insert member is greater than that of the clamping member. In this way, the optical waveguide device is reduced in anisotropy of compressive stress on the optical waveguide, so as to decrease the birefringence of the optical waveguide and thus effectively relax the polarization dependence.
The optical waveguide devices having the structures as described above (the optical waveguide devices according to the present invention) are fabricated by cementing the main member (first main member) and the sub member (first sub member) of the structure as described, together. In this fabrication method, first, the main member having a positive coefficient of linear expansion is prepared, the sub member having a negative coefficient of linear expansion is cemented to one major surface of the main member, and another major surface of the main member is polished or etched. The main member has a first major surface and a second major surface opposing the first major surface and incorporates an optical waveguide located between the first and second major surfaces.
In another fabrication method capable of fabricating the optical waveguide devices having the structures as described above, a sub member having a negative coefficient of linear expansion is prepared, and a main member incorporating an optical waveguide is formed on this sub member by a low-temperature CVD method.
These fabrication methods both are suitable for fabrication of the optical waveguide devices having the structures as described above (the optical waveguide devices according to the present invention). In general, an absolute value of the linear expansion coefficient of the main member is larger than that of the sub member. However, since the thickness of the main member itself can be made thinner by cementing the sub member to one major surface of the main member and thereafter polishing or etching the other major surface of the main member, the temperature dependence of optical characteristics of the optical waveguide in the optical waveguide device can be relieved to the practically negligible level. Since the thickness of the main member can be made thinner by forming the main member on the sub member by the low-temperature CVD method, the same effect can be achieved thereby.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.